Scientists seek global limits on damaging human activities

Climate change is but one of several challenges without borders confronting …

A major environmental disaster may no longer be a question of if, but when, according to a growing number of leading scientists. The multiple threats posed by unrestricted climate change, ozone depletion, and pollution, among others, has made the need for some form of global regulatory infrastructure more urgent than ever. Yet, as the recent G20 summit climate talks made resoundingly clear, the road there will be long, arduous, and full of half-hearted compromises.

This makes the latest call for a new model of global sustainability and restraint in Nature all the more remarkable—or hopelessly na�ve, depending on where you stand. A team of all-star researchers led by Stockholm University’s Johan Rockstr�m argues that we need a new approach to managing anthropogenic activities in order to create what they call a “safe operating space for humanity.”

Nine processes, nine boundaries

They propose to do so by identifying and establishing planetary boundaries for nine global processes that, if pushed beyond their tipping points, could result in large-scale environmental damage. They define the boundaries as the “values for control variables that are either at a ‘safe’ distance from thresholds—for processes with evidence of threshold behavior—or at dangerous levels—for processes without evidence of thresholds.”

Their description of climate change begins with a mention of the well worn “2�C guardrail” approach, the belief among most climate scientists that the rise in global average temperatures needs to be kept to, or below, 2�C above pre-industrial levels in order to avoid the worst environmental harm.

In practice, this means keeping atmospheric CO2 concentrations at or below 350 parts per million (ppm) by volume and restricting radiative forcing, the difference between incoming and outgoing radiation energy, to 1 watt per square meter (W/m2) or less above pre-industrial levels. Unfortunately, current CO2 levels stand at 387 ppm and the change in radiative forcing is 1.5 W/m2. Rockstr�m and his colleagues cite concerns about climate model uncertainties, feedback processes, the stability (or lack thereof) of ice sheets, and, of course, the rapid retreat of Arctic sea ice in quantifying their threshold.

Although it is reasonable to expect a certain number of species extinctions as the natural outcome of evolution and ecosystem dynamics, it is hard to argue with the fact that the rise of humanity has greatly accelerated this process. As the authors point out, species are becoming extinct at a rate that has not been seen since the last mass extinction event tens of millions of years ago.

In the past, marine species went extinct at a rate of 0.1 to 1 extinctions per million species per year, while mammals went extinct at a rate of 0.2 to 0.5 extinctions per million species per year. The current rate of extinction for all species is 100 to 1,000 times greater than these background rates. By the end of the century, close to 30 percent of all mammals, birds, and amphibians could be near extinction.

Unlike climate change, for which emission levels or CO2 concentrations can be capped at a particular value (if not in reality, at least in theory), setting a “boundary” for biodiversity loss is not clear-cut. An ecosystem’s composition and dynamics can become so deeply interwoven with a variety of natural processes that the removal of even a single species risks collapsing the entire structure.

Accurately predicting ecosystem resilience at a global (or even regional) level remains beyond the scope of science at the moment, so the authors recommend using extinction rate as the next best alternative to a true biodiversity model. More specifically, they suggest an extinction rate of 10 times the background rates.

To reduce excess levels of reactive nitrogen, the environmentally harmful form nitrogen assumes when it is converted for human use (primarily as fertilizer), they recommend a target of 35 million tons per year, or about 25 percent of the amount that is produced every year. Reactive nitrogen is the scourge of marine systems, where, when it accumulates in large quantities, it can cause prolonged periods of eutrophication and anoxia. These noxious conditions, which strip the ecosystem of all life, induce the formation of the appropriately named “dead zones.”

By the same token, the authors recommend restricting the flow of phosphorus, which, in sufficient amounts, can also contribute to the formation of dead zones, into the ocean to no more than 11 million tons per year. Phosphorus, unlike nitrogen, is mined from rock and, as a result, there is much less of it to go around. Of the 20 million tons of phosphorus that are mined every year, about 8.5 to 9.5 million tons of it winds up in the ocean. Though a far cry from the levels of reactive nitrogen that enter the ocean, this rate of influx is still 8 times higher than the natural background rate.

A good start, but caveats aplenty

In many respects, this study leaves more questions unanswered than it settles. How static are the boundaries that they have laid out? Who’s to say these won’t have changed in another year or less? What happens to these thresholds when those of the other processes are exceeded?

Also, the authors’ choices for boundaries run into a few snags. For one thing, many of these processes lack well-defined thresholds, as they themselves recognize, nor can they really because of the rapidly fluctuating nature of the climate system. Further complicating the picture is the fact that many of these thresholds can cross over, meaning that a slight alteration in one can trigger a disproportionate change in another—with potentially disastrous consequences (think an abrupt climate change). Moreover, most of these processes are controlled by more than one variable (the typical one being carbon dioxide concentration), making predictions tentative at best.

Putting aside its multiple layers of uncertainty, this study serves as an important first step in a long process to create a new global model of sustainability. The more relevant question now, of course, is whether humanity will rise to the challenge.